Ferrous bath-smelting processes with direct injection of ore and reducing agents are attractive from a commercial point of view. This arises from the fact that ore fines can be fed directly, obviating the need for lump feed as required by the blast furnace. Further, steaming coal can be used as reducing agent without prior removal of volatile matter in coke ovens.
Reducing agent (usually coal) consumption on such a system can be excessive, however, and the key to efficient operation of the smelting vessel is post-combustion of smelter offgas. In one embodiment of this concept, an oxygen-containing gas is admitted into the space above the bath and some of the carbon monoxide and hydrogen present is oxidised to carbon dioxide and water. The heat thus liberated is transferred back to the bath, and this reduces the amount of reducing agent needed in the process. Such a system is described in Australian patent specification No. 74409/81 the contents of which are incorporated herein by reference.
The smelter offgas from such a process contains appreciable amounts of carbon monoxide and hydrogen, and leaves the main vessel at relatively high temperature (eg. 1600.degree. C.). To further reduce the amount of reductant consumed in the overall process, it is beneficial to pre-reduce incoming iron ore feed with this offgas prior to admitting it to the smelting vessel. One such proposal is described in Australian patent specification No. 28044/84, in which the pre-reduction occurs in a shaft furnace.
Stickiness of particles under pre-reduction conditions is a potential problem. At temperatures above about 1000.degree. C., particles begin to cling to surfaces and thus create serious difficulties in plant operation. To avoid this, it is necessary to arrange the process in such a way as to maintain pre-reduction temperatures in the range 800-1000.degree. C. At temperatures much below 800.degree. C., pre-reduction rates slow appreciably and excessive residence-times are needed.
The proposal in the aforementioned AU-28044/84 aims to avoid this difficulty by cooling the offgas in a preconditioning chamber in which the offgas is additionally reduced by an intensive mixing action between the gas and coal dust caused by a special grid or lattice-lining over which the gas is passed. A cyclone is then required to separate the coal dust from the reduced, cooled offgas, prior to passing the gas to a shaft furnace to reduce the iron ore. However, it would be anticipated that a buildup of the sticky particles would occur on the special grid or lattice-lining and other surfaces of the preconditioning chamber.
Australian patent specification No. 69827/87 proposes an alternative arrangement in which fine ore having a particle size up to a maximum of 0.1 mm is introduced directly into the hot reducing gas immediately downstream of the offgas outlet from the smelting vessel.
It is an object of the present invention to alleviate the disadvantages of the aforementioned proposals and particularly to facilitate the partial reduction of particulate iron oxide containing material in the smelter offgas in an improved range of particle sizes.